1. Field of the Invention
This invention relates to a method for the reduction, preferably, the prevention, of precipitation of asphalt from crude oil.
2. Description of the Prior Art
Primary production of petroleum hydrocarbons from oil bearing formations is usually accomplished by drilling into or through the oil-bearing strata. Oil then flows into the bore hole, from which it is recovered by pumping to the surface. In most formations, primary recovery or primary production recovers no more than about 25 to 35% of the oil in the formation. A number of techniques have been developed whereby additional oil can be recovered from these formations. Known collectively as secondary or enhanced oil recovery (EOR) techniques, these methods can increase production substantially. In some cases, recovery as high as 60 to 65% has been achieved.
When EOR becomes necessary, a number of the wells in a producing field are designed as injection wells, and a fluid such as water, solvent or a gas is charged to these wells under pressure sufficient to force residual oil out of the formation into the remaining production wells, whence it is pumped to the surface. This method is referred to as "flooding." When a fluid such as liquid natural gas or liquid CO.sub.2 is employed in flooding, the method is referred to as "miscible flooding."
A serious problem, encountered with both primary and enhanced oil recovery, is the precipitation of asphaltenes and associated solid components (collectively referred to as asphalt), either within the oil-bearing formations or in the production borehole. This problem is encountered in at least one of its forms in virtually all oil fields since virtually all crude oil contains some amount of asphalt either in solution or in a colloidally dispersed state.
The asphalt remains dissolved or suspended as the case may be, so long as the oil remains subject to the ambient conditions found in its geological reservoir. A change in those conditions, however, can lead to harmful precipitation. Thus, precipitation of asphalts is frequently experienced within the oil-bearing formation during EOR when the crude oil is contacted with the miscible flooding solution. Even more frequently, precipitation occurs in the borehole when the oil is subjected to lower ambient temperature/pressure conditions. Precipitation in either location can lead to costly interruptions in production from the well while cleanup takes place.
Precipitated asphalt cleanup is usually accomplished by mechanical or chemical cleaning or by manipulating reservoir conditions (e.g. pressure or production rates). The approach to date has always been a remedial one and invariably requires the well or the formation to be out of production a period of time. There is great need for a preventive approach.
It is the objective of this invention to provide a method of preventing or at least reducing the precipitation of asphalt from crude oil.